In the cable industry, it is well known that changes in ambient conditions lead to differences in vapor pressure between the inside and the outside of a plastic cable jacket. This generally operates to diffuse moisture in a unidirectional manner from the outside of the cable to the inside of the cable. Eventually, this will lead to an undesirably high moisture level inside the cable, especially if a plastic jacket is the only barrier to the ingress of the moisture. High moisture levels inside a cable sheath system may have a detrimental effect on the transmission characteristics of the cable.
Furthermore, water may enter the cable because of damage to the cable which compromises its integrity. For example, lightning or mechanical impacts may cause openings in the sheath system of the cable to occur, allowing water to enter, and, if not controlled, to move longitudinally along the cable into splice closures.
Lately, optical fiber cables have made great inroads into the communications cable market. Although the presence of water itself within an optical fiber cable is not detrimental to its performance, passage of the water along the cable interior to connection points, terminals or associated equipment may cause problems and should be prevented. Further, in some climates, the development of ice within an optical fiber cable may have a crushing influence on the optical fibers in the core which may affect adversely the attenuation thereof.
In the prior art, various techniques have been used to prevent the ingress of water through the sheath system of a cable and into the core. Presently, many commercially available cables also include a water-swellable tape. The tape is used to prevent the travel of water through the sheath system and into the core as well as its travel longitudinally along the cable to closures and termination points, for example. Such a tape generally is laminated, including a water-swellable powder which is trapped between two polyester tapes. The water-swellable powder comprises a superabsorbent polymer (SAP).
Superabsorbent polymer materials generally are made in several ways which result in crosslinked polyacrylates, the major functional groups of which are carboxylate groups. Superabsorbent polymers may be made through a process of a crosslinking water-soluble polymers. Crosslinking renders the polymers insoluble in water and forms a matrix in which water is absorbed and retained. The amount of crosslinking is important and must be maintained at an optimum level depending on the application, such as the rate of water absorption and the total amount of water absorbed, for example. The amount of crosslinking determines the space in the network (matrix) of the superabsorbent polymer and thus the total volume of superabsorbent polymer, which in turn influences the density of functional groups in the network.
The mechanism by which a superabsorbent polymer absorbs and retains water can be described in two ways, physical and chemical. On the physical level, aqueous fluid wets the surface of the superabsorbent polymer and is physically distributed into and throughout the network of the superabsorbent polymer.
Chemical absorbency occurs on the molecular level. The aqueous fluid interacts with polymer chains. A carboxylate group will absorb water through a mechanism which is referred to as hydrogen bonding. The bulk of fluid chemically bonded to the superabsorbent polymer does not easily escape out of the network of the superabsorbent polymer.
Currently used superabsorbent polymers are not effective in a physiologically saline solution of about 0.9% NaCl or sea water due to the molecular structure of presently used superabsorbent polymers which have predominately carboxylate groups. For example, water absorbency drops from about 900 ml/g to 70 ml/g in a 0.9% NaCl solution and to 8 ml/g in a synthetic sea water solution.
The reasons why prior art superabsorbent polymers will not work well in a sea water environment, for example, are twofold. Carboxylates are sensitive to sodium chloride or other electrolytes. As a result, the water absorbing capability of prior art superabsorbent polymers decreases substantially when the polymers are exposed to salt.
The reason for such decrease may be explained in terms of a parameter termed osmotic pressure. Osmotic pressure is one of the mechanisms by which superabsorbent polymers absorb water because superabsorbent polymers are polyelectrolytes. An osmotic pressure gradient between the network of superabsorbent polymer and the surrounding aqueous solution determines the absorbency of the superabsorbent polymer. The osmotic pressure gradient between the network of the superabsorbent polymer and the surrounding aqueous fluid drives water into the network of the superabsorbent polymer.
The reduced water absorbing capability when the polymers are exposed to salt occurs because of a drop in the osmotic pressure gradient between the network of superabsorbent polymers and the external salt solution. When the surrounding aqueous solution changes from distilled water (zero concentration in functional groups or electrolytes) to 0.9% NaCl and then to sea water (about 3% in various electrolytes), the concentration gradient decreases, thus causing the osmotic pressure gradient to decrease. The decrease in osmotic pressure gradient results in a decrease in absorbency of the superabsorbent polymer. Another reason for the reduction in water absorbing capability is an effect referred to as the common ion effect which also decreases the osmotic pressure.
Another reason for the substantial decrease in absorbency by prior art superabsorbent polymers in salt solutions is multivalent ion complexation. In a multivalent ion environment, the multivalent ion will complex with the carboxylates and limit polymer chain extension and charge repulsion between the carboxylate ions which in turn reduces water absorbency. A well known phenomena in water soluble polymer chemistry is referred to as salting-out. A carboxylate-containing water soluble polymer solution can be turned easily into a precipitation of polymer separated from aqueous solution by the addition of a multivalent ion such as calcium. Calcium and other multivalent ions exist in sea water in low concentrations. The multivalent charges on the calcium ion will attract and complex with carboxylates in polymer chains. In effect, the complexation of these polymer chains increase the molecular weight of the polymer which becomes insoluble in water and precipitates out of solution.
In the case of a carboxylate-containing superabsorbent polymer, the net effect of complexation of multivalent ions with carboxylates is crosslinking. The addition of such complexation or crosslinking upsets the optimum level of crosslinking introduced when superabsorbent polymers are made. Therefore, the absorbency of the superabsorbent polymer is altered accordingly. This occurs in addition to the aforementioned drop in osmotic pressure gradient when superabsorbent polymers are exposed to any salt solution, not necessarily multivalent salts.
Another property of superabsorbent polymers is the temperature tolerancy thereof. The repetitive application of heat energy to carboxylate groups, whether in highly humid or dry conditions, will form anhydrides. The formation of an anhydride from two carboxylate groups, in effect, is a crosslinking of polymer chains which, as mentioned hereinabove, can reduce the water absorbency of the superabsorbent polymer. In some cases, high temperature and/or high humidity will destroy the crosslinking of polymer chains, thus reducing the water absorbency of the superabsorbent polymers. In either case, the optimum level of crosslinking can no longer be maintained and, accordingly, the water absorbing property of the superabsorbant polymer is changed.
What is sought after and what seemingly is not available is a cable that includes a superabsorbent polymer which is effective notwithstanding exposure to salt solutions such as, for example, in cables which are deployed in sea water environments or in hygienic and agricultural products which are exposed to saline environments. Further, the sought after superabsorbent polymer which may be included in cables should be one which is substantially temperature insensitive.